Thermoplastic olefins (TPOs), impact copolymers (ICPs), and thermoplastic vulcanizates (TPVs), collectively referred to herein as “toughened polymer blends”, typically comprise a crystalline thermoplastic component and a high molecular weight or crosslinked elastomeric component. These toughened polymer blends often have multiphase morphology where the thermoplastic component, such as isotactic polypropylene (often referred as the hard phase), forms a continuous matrix phase and the elastomeric component (often referred as the soft phase), generally derived from an ethylene containing copolymer, is the dispersed component. The polypropylene matrix imparts tensile strength and chemical resistance to the blends, while the ethylene copolymer imparts flexibility and impact resistance. For some compositions the elastomeric phase is the continuous matrix phase, and the hard phase is dispersed. These soft blends are known as thermoplastic rubbers (TPRs).
TPOs and TPVs are typically mixed in extruders and contain no cross-products between the blend components. Although ICPs can also be produced by mechanical blending, today most are made as in-reactor blends. Porous isotactic polypropylene particles are produced in a slurry polymerization process in a first stage, and the particles are coated with ethylene-propylene rubber in a second gas phase polymerization stage. Such in-reactor blending of ICPs is generally preferred since in-reactor blends not only provide lower production costs but also offer the possibility of improved mechanical properties through more intimate mixing between the hard and soft phases. Cross-products can also be formed in the ICP process but are at very low levels because the EP rubber is polymerized in the gas phase while the iPP is already a solid.
Datta, et al [D. J. Lohse, S. Datta, and E. N. Kresge, Macromolecules 24, 561 (1991)] describe EP backbones functionalized with cyclic diolefins by terpolymerization of ethylene, propylene and diolefin. The statistically functionalized EP “soft” block is then copolymerized with propylene in the presence of a Ziegler-Natta catalyst capable of producing isotactic polypropylene. In this way, the authors speculate that some of the “hard” block polypropylene chains are grafted through the residual olefinic unsaturation onto the EP “soft” block previously formed. No NMR, GPC-3D, or small angle oscillatory shear data are presented in this paper to prove the existence of cross-products. The paper reports the presence of materials that extract at intermediate temperatures between EP (boiling hexane) and iPP (boiling xylenes), but more recent work has shown that these products cannot be EP-iPP cross-products, since the latter extract with the iPP. See also, EP-A-0 366411.
U.S. Pat. No. 4,999,403 describes similar graft copolymer compounds where functional groups, such as amines or alcohols, in the EPR backbone are used for grafting isotactic polypropylene having maleic anhydride reactive groups. The synthesis of these functionalized EPRs and iPPs involves polar reagents, and the final product contains polar bonds. The graft copolymers are said to be useful as compatibilizer compounds for blends of isotactic polypropylene and ethylene-propylene rubber. A limitation of this class of reactions, in which chains with multiple functionalities are used in subsequent reactions, is the formation of undesirable high molecular weight material typically referred to as gel in the art.
U.S. Pat. Nos. 5,504,171 and 5,514,761 disclose α-olefin/α,ω-diene copolymers, which are generally crystalline, free of gel and crosslinks, and contain unsaturated side chains and long chain branching. The copolymers contain up to 5 mole percent diene incorporated therein, and may be prepared by copolymerization using a solid-phase, insoluble coordination catalyst, such as Ziegler-type catalyst, without a solvent, and below the crystalline melting point of the copolymer. Soluble catalysts, such as, a biscyclopentadienyltitanium(IV)dichloride-aluminum alkyl system, are said to be generally unsuitable because the second terminal bond of the α,ω-diene is not protected from the catalyst and remains available for reaction in a gel-forming polymerization reaction. Cross-products could not form in these polymers, because a second polymerization stage is not used.
U.S. Pat. Nos. 6,660,809, 6,750,307 and 6,774,191 disclose a branched olefin copolymer having an isotactic polypropylene backbone, polyethylene branches and, optionally, one or more comonomers, but no dienes. The total copolymer is produced by a) copolymerizing ethylene, optionally with one or more copolymerizable monomers, in a polymerization reaction under conditions sufficient to form copolymer having greater than 40% chain end-group unsaturation; b) copolymerizing the product of a) with propylene and, optionally, one or more copolymerizable monomers, in a polymerization reactor under suitable polypropylene polymerization conditions using a chiral, stereorigid transition metal catalyst capable of producing isotactic polypropylene; and c) recovering the branched olefin copolymer.
One of the problems with existing methods of producing in-reactor polymer blends is that the amount of cross-products tends to be very low, typically less than 5 mole % of the overall blend. Since these cross-products frequently influence the material flow and mechanical properties of the final blend, it would be desirable to be able to control the amount of the cross-products and particularly to increase the amount to high levels, such as at least 20 mole %, preferably at least 50 mole %, and most preferably 100 mole %.
According to the invention, there is provided a novel method of producing in-reactor polymer blends, which allows control of the production of cross-products up to a high level, and novel in-reactor polymer blends produced by such method.